| Paracrine signaling is important for tissues both during embryonic development and in regulation of the tissue's cell lineages in adults. In liver, the mesenchymal cells adjacent to hepatic parenchymal cells secret soluble factors as well as extracellular matrix (ECM) molecules that regulate hepatic parenchymal cells to grow and/or to differentiate in order to form a functional liver. Human hepatic stellate cells (hHpSTCs) with classic stellate cell markers (vitamin A+, desmin+ and alpha-smooth muscle actin+) and liver sinusoidal endothelial cells (VEGFr +, von Willebrand factor+ and CD31+) are two major mesenchymal cell types in the liver. They were found present in primary cultures of human hepatic stem cells (hHpSCs) derived from fetal livers and were found to be critical as "companion cells" for survival and expansion of the hHpSCs. Many of the markers are shared by hHpSCs and the other pluripotent hepatic progenitors in human livers, hepatoblasts (hHBs) including expression of albumin, epithelial cell adhesion molecule (EpCAM), cytokeratins 8, 18 and 19, hedgehog proteins, and telomerase but not hematopoietic markers (CD45, CD34, CD38) or mesenchymal markers (those of endothelia or HpSTCs). However, they can be distinquished in that hHpSCs express neuronal cell adhesion molecule (NCAM) and claudin 3 but are negative for intercellular adhesion molecule-1 (ICAM-1) and a-fetoprotein (AFP), whereas hHBs express intercellular cell adhesion molecule (ICAM-1), AFP, higher levels of albumin but not claudin 3 or NCAM.;The hHpSCs behave differently depending on type of feeder cells are used. Feeders of angioblasts or endothelia resulted in maintenance of hHpSCs as stem cells; feeders replete with HpSTCs resulted in lineage restriction of hHpSCs to hepatoblasts; and STO feeders caused even more differentiation to both hepatoblasts and committed progenitors.;Representative feeders of angioblasts/endothelia versus HpSTCs were characterized for expression of matrix genes and proteins; those matrix components identified were then tested as substrata for effects on the hHpSCs. The hHpSCs remained as stem cells on substrata of type III collagen or culture plastic; they differentiated into hepatoblasts on substrata of laminin or on top of collagen type IV; they differentiated into hepatoblasts and committed progenitors on the surface of type I collagen and into hepatocytes when embedded into type I collagen gel.;STO feeder cells secret inflammatory cytokines and factors, many of them identified in other studies as being produced following liver injury. A number of the factors were induced to be secreted to higher levels by co-culture of STO cells with human hepatic progenitors. These included several members of the interleukin family (e.g. IL-4, IL-5, IL-6, IL-10, IL-11 and IL-13), eotaxin, transforming growth factor-beta1 (TGF-beta1), interferon-gamma (IFN-gamma), macrophage inflammatory protein-2 (MIP-2), and others. Some of these factors have been found in preliminary studies to have potent effects on human and rodent hepatic progenitors but have not yet been fully characterized. They are assumed to play complementary roles to those of the matrix components in regulating hHpSCs. Analyses of these role(s) will be part of future studies. |
